game design – Physics-based marble race: how can you minimize the gap between first and last?

In building a 2d sideview marble game, where they race down the track via gravity. The problem is once one gets ahead of the others, it usually wins.

In racing games they can address this by subtly changing the enemies speed or ai skill, or power ups but I can’t do that. I’m looking for obstacles I can add to address this.

The only things I can think of are:

Gate – holds front players in place until a certain amount catch up, then releases

Spring – launches marbles up or back, and they have to land just right to continue on the track

Are there any other methods I could use?